Optically anisotropic films having negative retardations in the thickness directions have functions of increasing viewing angle of liquid crystal display devices and improving light use efficiency of brightness increasing films, and thereby have been variously studied. Particularly methods for aligning liquid crystal molecules perpendicularly and fixing the resulting alignment state have been actively studied from the viewpoints of easiness and uniformity of production.
For example, as reported in JP-A-05-53104, JP-A-05-27235, JP-A-04-16916, and JP-A-09-101515, viewing angle of TN and STN liquid crystal display devices can be improved by using methods comprising the steps of heating a high-molecular liquid crystal at the glass-transition temperature (Tg) or higher to perpendicularly align the liquid crystal molecules in the liquid crystalline phase and rapidly cooling the molecules to form a retardation film.
As methods for fixing the perpendicularly aligned state of the liquid crystal, methods comprising the steps of aligning a low-molecular, polymerizable liquid crystal having a polymerizable group such as an acryloyl group and irradiating the liquid crystal with UV or an electron beam to polymerize and fix the liquid crystal are also proposed (JP-A-05-142531 and JP-A-08-21915).
Additionally methods comprising aligning and fixing a polymerizable liquid crystal oligomer having properties between the above liquid crystal materials are proposed (JP-A-07-230086 and JP-A-07-294903).
Further, examination of using these optically anisotropic films in combination with stretched optically anisotropic films to improve viewing angle of STN liquid crystal display devices is reported (JP-A-07-230086, JP-A-07-294903, JP-A-02-256023, JP-A-05-241019, and JP-A-08-21999).
Also reported are effects of the optically anisotropic films for increasing light use efficiency of brightness increasing films using cholesteric liquid crystals (JP-T-2000-514202, JP-T-2001-500276, and JP-A-2003-177242, the term “JP-T” as used herein means a published Japanese translation of a PCT patent application) and for reducing viewing angle dependence of polarizing plates to improve viewing angle of in-plane switching mode liquid crystal display devices (JP-A-10-307291 and JP-A-11-133408).
Though liquid crystal display devices using the above mentioned optically anisotropic films have contrast improving effects, they are disadvantageous in undesired coloration of a display image and contrast reduction at a more oblique angle.
As a result of research in view of the disadvantages, the inventor has found that the main cause for the problems is such that the phase differences of the conventional optically anisotropic films are increased at shorter visible light wavelengths. While the optically anisotropic films have retardations within desired ranges at a certain monochromatic light wavelength, they have different retardations at the other wavelengths in the visible region.
Thus, in the case of white light containing lights having various wavelengths, phase angle polarization of the films varies depending on the wavelengths, and the white light is converted to colored polarized lights.
This problem is caused because a material forming the retardation plate has a wavelength dispersion of retardation.
Proposed as a solution for the problem in JP-A-2002-267838, etc. are methods of mixing a rod-shaped liquid crystal with molecules to be aligned perpendicularly to the major axes of the rod-shaped molecules and applying the obtained liquid crystal composition with a reciprocal wavelength dispersion.
However, the molecules mixed with the rod-shaped molecules have no liquid crystalline properties, and the composition loses its liquid crystalline properties when the content of the molecules is increased, whereby it is difficult for the methods to control the wavelength dispersion.